Vibrations are caused in the normal operation of various types of rotating machinery, for example, aircraft turbine engines, rotating devices deployed on-board satellite and other spacecraft for attitude adjustment purposes, such as control moment gyroscopes and reaction wheels, automobile turbochargers, generators and pumps, and the like. When machinery rotates, there can be loads or forces created due to imbalances in the rotation, causing synchronous vibrations. In addition, rotating machinery can also experience non-synchronous vibrations, often the result of destabilizing forces and/or an unstable rotor-bearing system. Non-synchronous vibrations may result in excessive rotor response and/or excessive noise, both of which are unacceptable because they produce unacceptable bearing loads, engine vibration, and/or audible noise. Non-synchronous vibrations may have a detrimental effect on both the performance of the rotating machinery and the comfort of persons who may be present on-board a vehicle associated with the rotating machinery (e.g., airplane, helicopter, automobile, etc.). For example, the vibrations can travel through the frame of an airplane to compartments containing passengers and operators. While not harmful, vibrations and the attendant noise can be an unpleasant aspect of travel for the occupants.
Bearings are used between rotating and stationary components of machinery to reduce friction therebetween and support the rotating component, and to extend the useful lives of the machinery. Use of oil-free bearings, such as foil bearings, removes the need for an oil lubrication system and provides other significant benefits. Foil bearings are currently used in many applications, both terrestrial and in aerospace. Foil bearings have been used in a rotor support structure to support a rotating shaft in a wide range of turbomachinery. The foil bearing supports a load of the rotor shaft via a fluid film formed as a result of the rotor shaft rotation, and thus may be called a hydrodynamic foil bearing. Bearing stiffness is a significant factor in providing support as the shaft rotates relative to the stationary components. Conventional foil bearings exhibit isotropic stiffness, i.e., the stiffness in the first support direction is the same in magnitude as the stiffness in the second support direction. Stiffness, K, is the rigidity of the foil bearing (or bearing housing), i.e., the extent to which it resists deformation in response to an applied force (F). The higher the stiffness, the less compliance (i.e., deflection) there will be when loads or destabilizing forces are applied, thereby affecting control of non-synchronous vibrations. Conventional foil bearings lack the ability to provide effective non-synchronous vibration isolation.
Accordingly, it is desirable to provide rotor support structures including anisotropic foil bearings or anisotropic bearing housings and methods for controlling non-synchronous vibration using the same. It is also desirable for the anisotropic foil bearings and anisotropic bearing housings to be lightweight and capable of being retrofitted into existing rotating machinery, such as a gas turbine engine. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the present invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.